Real World Camera Raw: Image Editing and Image Degradation

No matter what you do, edits degrade the data in an image file in three different ways: clipping, and tonal range expansion and contraction. Jeff Schewe shows you how to make the best use of the available bits you’ve captured to produce the desired image appearance while preserving as much of the original data as possible.

From the book

From the book

Just about anything you do to change the tone or color of pixels results in some kind of data loss. If this sounds scary, rest assured that it’s a normal and necessary part of digital imaging. The trick is to make the best use of the available bits you’ve captured to produce the desired image appearance while preserving as much of the original data as possible. Why keep as much of the original data as possible if you’re going to wind up throwing it away later? Very simply, it’s all about keeping your options open.

The fact is, you don’t need a huge amount of data to represent an image. But if you want the image to be editable, you need a great deal more data than you do to simply display or print it. Figure
2-5 shows two copies of the same image. They appear similar visually, but their histograms are very different. One contains a great deal more data than the other.

Despite the vast difference in the amount of data they contain, it’s hard to see any significant differences between the two images—you may be able to see that the one with more data shows more details on the chest feathers, but it’s a pretty subtle difference. Figure
2-6 shows what happens when a fairly gentle curve edit is applied to the images shown in Figure
2-5. The difference is no longer subtle!

Here you see the images from Figure
2-5 after application of a fairly gentle S-curve (to increase contrast slightly) to both images. The differences between the data-rich (left) and data-poor (right) versions are now much more obvious. The data-poor version shows much less detail, and displays both exaggerated contrast and unwanted hue shifts.

The difference between the two images is in the way they were edited. The one with the larger amount of data made full use of Camera Raw to convert the raw file into a 16-bit/channel image in Photoshop. Additional edits were done in 16-bit/channel mode. The one with the smaller amount of data was converted to an 8-bit/channel image at camera default settings, and the edits were performed in 8-bit/channel mode in Photoshop.

Losing Data and Limiting Options

The sad truth is that every edit you make limits the options that are available to you afterward. You can keep many more options open by making full use of Camera Raw controls and by converting to a 16-bit/channel image rather than an 8-bit one. But no matter what you do, edits degrade the data in an image file in three different ways: clipping, and tonal range expansion and contraction.

Clipping. The black and white input sliders in Photoshop’s Levels command and the Exposure and Blacks sliders in Camera Raw are clipping controls. They let you force pixels to pure white (level 255) or solid black (level 0).

Depending on how you use the sliders in Photoshop or Camera Raw, you may clip some levels—in fact, it’s often desirable to do so. On the highlight end, you normally want to make sure that specular highlights are represented by level 255, so if the image is underexposed, you usually want to take pixels that are darker than level 255 and force them to pure white. But if you go further than that, you may clip some levels. For example, if you have pixels at levels 252, 253, and 254, and you set the white input slider in Levels to level 252, then all the pixels at levels 252, 253, and 254 are forced to 255. Once you make this edit permanent, the differences between those pixels are gone, permanently.

On the shadow end, you often want to clip some levels, because typically there’s a good deal of noise in the shadows. If everything below level 10 is noise, for example, it makes perfect sense to set the black input slider in Levels to 10, to force everything at level 10 and below to solid black. Again, you lose the distinction between the unedited levels 0 through 10 permanently, but it’s not necessarily a bad thing. Figure
2-7 shows how clipping works.

However, if you’re used to adjusting clipping in Photoshop’s Levels, you’ll find that the Exposure and Shadow controls in Camera Raw behave a bit differently from Levels’ black and white input sliders, partly because the latter works on linear-gamma data rather than the gamma-corrected data that appears in Photoshop, and partly because Camera Raw’s Exposure slider can make negative as well as positive moves.

If the camera can capture the entire scene luminance range, as is the case with the image in Figure
2-7, it’s usually best to adjust the Exposure and Blacks sliders to near-clipping, leaving a little headroom (unless you actually want to clip to white or black for creative reasons). If the camera can’t handle the entire scene luminance range, you’ll have to decide whether to hold the highlights or the shadows, and your choice may be dictated by the captured data: if highlights are completely blown, or shadows are completely plugged, there isn’t much you can do about it in the raw conversion. See the sidebar “How Much Highlight Detail Can I Recover?” later in this chapter.

Tonal range compression. When you compress a tonal range, you also lose levels, in a somewhat less obvious way than you do with clipping moves. For example, when you lighten the midtones without moving the white clipping point, the levels between the midtone and the highlight get compressed. As a result, some pixels that were formerly at different levels end up being at the same level, and once you make the edit permanent, you’ve lost these differences, which may potentially represent detail. See Figure
2-8.

When you use the Brightness slider in Camera Raw or the gray slider in Levels to brighten the midtones, you compress the highlights and expand the shadows. The images and histograms show Camera Raw’s Brightness control, and the histogram shows the results of using the gray input slider in Levels on an 8-bit/channel image. The gaps are from expansion, the spikes from compression.

Tonal range expansion. A different type of image degradation occurs when you expand a tonal range. You don’t lose any data, but you stretch the data that’s there over a broader tonal range, and hence run the danger of losing the illusion of a continuous gradation. Almost everyone who has used Photoshop for more than a week has encountered the experience of pushing edits just a little too far and ending up with banding in the sky or posterization in the shadows. It’s simply caused by stretching the data over too broad a range so that the gaps between the available levels become visibly obvious. See Figure
2-8.

If all this makes you think that editing images is a recipe for disaster, you’ve missed the point. You need to edit images to make them look good. Sometimes you want to throw away some data—shadow noise being a good example—and the inherent data loss is simply something that comes with the territory. It isn’t something to fear, just something of which you should be aware. The importance of the preceding information is that some editing methods allow you more flexibility than others.